1. Field of the Invention
The present invention relates to aircraft fuel systems; and more particularly to aircraft fuel systems having capabilities for strategically storing and distributing fuel about the aircraft in ways that combat stress induced on the airframe by wing-lift.
2. Background
The design process of an aircraft, particularly with respect to the interaction between the fuselage and wing structures, must account for the complex set of loads that will be experienced during operation both on the ground, and in the air, under fully laden conditions. These loads are the result of numerous factors including the airframe weight, the load being carried, and aerodynamic effects imposed upon the aircraft.
In many, if not most designs, the wing also serves as a fuel tank. Traditionally, wing tanks have been established by joining together and sealing certain structural members within the wing and thereby forming a single, large, fuel-carrying volume. The wing's upper and lower aerodynamic surfaces, front and rear spar, and the inner- and outer-most ribs typically define the periphery of a wing fuel tank. Such a tank is sometimes partitioned into a series of smaller, interconnected fuel cells through the presence of baffle ribs placed along the wing chord. To prevent the sudden movement of large fuel quantities during aircraft maneuvers, such baffle ribs can be configured to restrict the migration of fuel from one side of the rib to the other. For positive dihedral wings, gravity continually draws the fuel toward the wing root where the fuel pumps are located and which deliver the fuel from the fuel tank to the aircraft's engine(s) as schematically represented in accompanying FIGS. 2a–2c. 
For a given wing design, increasing the aircraft's maximum allowable weight results in an increase in the structural bending moments at the wing root as depicted in accompanying FIGS. 3a–3b where it is illustrated that when aircraft weight is increased, lift on the craft must be commensurately increased to achieve flight. Modifying the wing's primary structure to accommodate the increased loads requires a significant amount of time, cost and effort. As a result, any wing modification must be considered carefully before being initiated. To preclude the burden of modifying a wing's primary structure, design margins are typically “built-in” during the original design phase to allow for future aircraft growth.
A net effect that results in a decrease of wing bending can be achieved by strategically locating mass toward the wing tip to counteract the wing's aerodynamic up-bending as is schematically represented in FIGS. 4a–4c. The premise of the solution of FIG. 4a is embodied in U.S. Pat. No. 2,585,480 where additional mass is taught to be added by the installation of weighty materials that can be moved toward and away from the wing tip. While effective for reducing the upward bending moment, this approach results in reduced aircraft operational capability since an equivalent mass reduction in fuel and/or cargo must be observed.
Another solution is to include multiple, discrete fuel tanks in the wing, and to first utilize the fuel in the most-inboard tank(s) as is schematically represented in FIG. 4b. Another related solution is to have an additional tank at the wing tip in the manner that is schematically represented in FIG. 4c. In both instances, outboard-fuel is maintained to provide the desired structural benefit, while the inboard-fuel is first consumed by the engine(s). During the later stages of the flight mission, the fuel within the outboard tank will be consumed. The use of multiple fuel tanks within a wing has the detrimental effect of increasing parts count, system complexity, failure effects, and pilot workload.